Recently, there has existed increasing interest in energy storage technology. Batteries have been widely used as energy sources in portable phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development for them. In this regard, electrochemical devices are the subject of great interest. Particularly, development of rechargeable secondary batteries is the focus of attention. Recently, continuous studies have been performed to develop a novel electrode and battery having an improved level of capacity density and specific energy.
Among the currently used secondary batteries, lithium secondary batteries, developed in early 1990's, have a higher drive voltage and energy density than those of conventional batteries using aqueous electrolytes (such as Ni-MH batteries, Ni—Cd batteries and H2SO4—Pb batteries), and thus are spotlighted in the field of secondary batteries. However, lithium secondary batteries have a problem related to their safety, due to ignition and explosion caused by the use of an organic electrolyte. Also, lithium secondary batteries have a disadvantage in that they are obtained via a relatively complicated manufacturing process.
Evaluation of and security in safety of batteries are very important. It should be considered in the first place that users have to be protected from being damaged due to malfunctioning of batteries. To satisfy this, safety of batteries is strictly restricted in terms of ignition and combustion in batteries by safety standards. Therefore, many attempts have been made to solve safety-related problems of batteries.
In order to prevent heat emission from batteries, various methods including use of a protection circuit, use of heat occlusion by a separator, etc., have been suggested. However, use of a protection circuit causes limitation in downsizing and cost reduction of a battery pack. A mechanism of heat occlusion by a separator often acts inefficiently, when heat emission is generated rapidly. Recently, use of organic electrolyte additives has been also suggested to solve the above-mentioned problems. However, safety mechanisms based on electrolyte additives have disadvantages in that calorific value (J) may vary depending on charging current or internal resistance of a battery, and timing is not uniform. Therefore, such safety mechanisms are always followed by degradation in the overall quality of a battery.
Meanwhile, thermal unstability of a lithium secondary battery is largely caused by side reactions between a cathode active material and an electrolyte. In other words, a cathode shows a very unstable structure in a charged state, particularly at high temperature. The cathode having an unstable structure in a charged state causes a rigorous exothermic reaction with an electrolyte, such reaction being followed by the structural collapse, resulting in liberation of oxygen. Thus, such exothermic reaction and oxygen liberation cause severe heat emission in a battery, and the battery may explode finally. Therefore, it is important to control the reaction heat generated by such side reactions between a cathode and an electrolyte in order to improve the safety of a battery. Under these circumstances, intensive research and development for a novel cathode active material has been conducted (Japanese Patent Publication No. 08273665 and U.S. Pat. No. 02/07251). In the case of the cathode active materials disclosed in the above patents, it is possible to control the exothermic reaction to a certain degree, as compared to other cathode active materials according to the prior art. However, the cathode active materials have disadvantages in that they are always followed by degradation in the quality or capacity of a battery.